For many years computer tape drives have had heads and circuits which allow tapes to be read backwards as well as forwards, as illustrated by U.S. Pat. No. 3,947,876 entitled "Dual Density 800 bpi NRZI and 1600 bpi PE Read Circuit for a Digital Magnetic Tape Transport," by Martin D. Gray. Mr. Gray's patent (C. J. Kennedy Company) provides that when reading tape backwards in a phase encoded format, high to low flux transitions, which create a negative peak on the read head signal for a one bit in the forward direction, appear as low to high flux transitions and create a positive peak at a one bit when reading tape in the reverse direction. This inversion of the read head signal also causes the differentiated read head signal to be inverted. For this reason, a phase encode reverse signal, PE-RVS, drives a second input exclusive OR to reinvert the squared, differentiated read head signal so that at the output of exclusive gate the squared, differentiated read head signal always has the same polarity with a low to high transition representing a one and a high to low transition representing a zero. It thus becomes possible for the same circuitry to process the squared, differentiated read head signal to detect binary zeros and ones regardless of tape direction.
While this patent described above a particular arrangement of a read circuit for a digital magnetic tape transport, there have been many others. A problem is that tape drives may not work in a particular environment, and some tape drives are able to read, in spite of the teaching of the art, only in a forward direction. The particular problem addressed here is how to make any SCSI tape drive such as a 9-track cartridge and like SCSI attached tape drives function with the IBM S/390 product "IBM RS/6000 and System/390 Server-on-board" regardless of the drive used. A solution to the problem needs to address the fact that many SCSI attached tape drives can read only in a forward direction.